1. Field of the Invention
The invention is directed to a hydraulic actuation device for the adjustment or positioning of an actuation element or an element to be operated in motor vehicles. The hydraulic actuation device includes a piston-cylinder unit having a cylinder housing and a piston axially movable in the cylinder.
2. Description of the Related Art
A prior art hydraulic actuation device including a piston-cylinder unit is known from DE 196 08 132 A1. This prior art device has a cylinder with a cylinder housing forming a cylinder space. In the preferred embodiment, the cylinder is a master cylinder in a hydraulic system in a motor vehicle. A piston is arranged so as to be displaceable axially in the cylinder space and a piston rod connected to the piston in a positive engagement is guided out of the cylinder housing. In addition to the above-described solution to the problem of connecting a piston with a piston rod, FIGS. 1 and 1a of the prior art reference show a cylinder space which has, along a surface line, a channel which extends parallel to the center axis of the cylinder housing and which, when the piston is displaced, produces a connection between the respective diminishing cylinder space and a primary seal to maintain the pressure ratios at the cylinder side of the primary seal at the pressure level of the medium to be compressed.
If the cylinder space and the pressure line system connected thereto are to be filled with a liquid medium which is required to be free of bubbles, difficulties are encountered if precautions are not taken. For example, the cylinder must be filled before the piston assumes its fully extended position so that a connection is formed between the cylinder space and the pressureless space located behind the primary seal. Using these precautions, any air bubbles present in the liquid may be guided to a compensation vessel through the pressureless space. However, once the prior art device is installed, it is difficult to release any air bubbles present in the liquid and may be impossible depending on the installed position of the cylinder.